A “semaphore” (also referred to as “critical sections” or “mutex”) is a hardware and software construct that allows coordination or synchronization of operations in which multiple processes compete for shared resources (e.g., memory, files). In general, a semaphore is a value that is stored in a designated location in operating system memory that processes can check and change. Based on the value of the semaphore, a process can either access the shared resource or wait for a period of time and check the semaphore again.
Semaphores in conventional computer systems are typically implemented as software routines using hardware support of atomic “test and set” or similar types of instructions (e.g., lock, bit test, bit test and set, bit test and reset). Using this semaphore implementation, a producer-consumer communication relationship can be established through shared (e.g., global) data and one or more semaphores. The semaphore allows shared data to be modified by a selected one of multiple processes that are attempting to modify the data, which provides data consistency.
This semaphore construct is “passive” because threads must perform polling operations to acquire a semaphore. The polling requirement consumes processor and system resources that could otherwise be used for other purposes. Therefore, traditional semaphores can result in inefficiencies.